Got 'Couch Potato' Teens? It's Not Helping Their Mental Health

"Our findings show that young people who are inactive for large proportions of the day throughout adolescence face a greater risk of depression by age 18," said study author Aaron Kandola, a psychiatry Ph.D. student at University College London (UCL).

"We found that it's not just more intense forms of activity that are good for our mental health, but any degree of physical activity that can reduce the time we spend sitting down is likely to be beneficial," he explained in a university news release.

"We should be encouraging people of all ages to move more and to sit less, as it's good for both our physical and mental health," Kandola added.

In the study, more than 4,200 participants in England wore devices that tracked their movement for at least 10 hours over at least three days when they were ages 12, 14 and 16.

They also completed questionnaires to assess depressive symptoms such as sadness, loss of pleasure and poor concentration.

Every additional 60 minutes of inactivity a day at age 12, 14 and 16 was associated with an increase in depression scores of 11%, 8% and 10.5%, respectively, by age 18.

Those with high levels of inactivity at all three ages had just over 28% higher depression scores by age 18.

The researchers tallied all activity: moderate-to-physical activity (running or cycling), and light activity, which included walking around, playing an instrument, or doing the dishes. They noted that falling activity scores through the years meant teens were doing less light activity and sitting more.

"Worryingly, the amount of time that young people spend inactive has been steadily rising for years, but there has been a surprising lack of high-quality research into how this could affect mental health," Kandola said. "The number of young people with depression also appears to be growing, and our study suggests that these two trends may be linked."

The study, which doesn't prove cause and effect, was published Feb. 11 in The Lancet Psychiatry journal.

"A lot of initiatives promote exercise in young people, but our findings suggest that light activity should be given more attention as well," said study senior author Joseph Hayes, from UCL Psychiatry.

"Light activity could be particularly useful because it doesn't require much effort and it's easy to fit into the daily routines of most young people … Small changes to our environments could make it easier for all of us to be a little bit less sedentary," Hayes said in the release.